Owners and managers of 7-Eleven stores in Virginia and New York have been charged with running an underground slave racket that mistreated immigrant workers, federal officials said.

The government seized the 14 stores after undocumented Pakistani workers were hired and given false identities culled from children and the dead, federal prosecutor Loretta Lynch said, in a practice dubbed by authorities as a "modern-day plantation" system.

Up to 50 Pakistanis were forced to work as much as 100 hours a week, for a fraction of their due wages, the investigation found. They also had to live in houses owned by storeowners.

"The 7-Eleven franchises . . . will be better known for their big fraud than their Big Gulp," James Hayes, a special agent in charge of investigations at the New York office of the Immigration and Customs Enforcement, told ABC News.

While the initial investigation, one of the largest ever for immigrant employment, covered workers in just two states, the federal investigation, conducted jointly by the Department of Justice and the Department of Homeland Security, now has expanded to 40 stores in seven states including Florida, New Jersey and Michigan, The Hill reported.

Southland Corp., which owns 7-Eleven, said the company is taking the matter seriously, opening an "aggressive" audit of all franchise employees. Southland said in a statement that it would assume corporate operations for all 7-Eleven stores involved in the federal investigation.

The company has more than 7,600 U.S. stores.

Nine people who owned and managed the stores cited in the criminal probe have been charged in the federal indictment with fraud, identity theft, and concealing illegal immigrants.

Many of the employees allegedly abused in the fraud now face immigration hearings and possible deportation, ABC News said.

A federal criminal probe into mistreatment of immigrant workers by several franchise owners of the convenience store chain 7-Eleven has led to officials charging them with running an underground slave racket.